Shock Cord Length Revisited or Shock Cord Length in Dual Deploy

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As I said, we had an oops with the electronics installation. The ARTS2 was installed upside down and did not detect apogee.



It was a group project...not an L3 project. 2 grams was used because that was more than enough to get the chute out of the cannon. To our surprise it was also enough to separate everything.

the ground test was in refrence to the other post by cbrarick of going from 3 to 6 grams on his l3 cause jim and tim told him it was best... not this project...

But, i wans't being critical of the group project.. OUCH on the upside down part, that stuff happens.....if you glue the wires to the slet or wherever they are mounted, you wont mount your altimeter upside down... (puting an avbay in upside down is another story... )

But, since you didnt answer the difference between 2 and 3 grams is thermal energy. Granted... i dont know what that difference is ... But, it does not equal more "eplosive damage", 2 and 3 grams are equally capable of that.
 
Not trying to be a (....?) here, but where's the method in the discussion?

I (or anyone ) can make 4 grams of 4F BP go Fawump! or make 2 grams of 4F BP go downright KA-BLAM!!!. A whole different critter when discussing the separation of rocket parts.

Then comes the 2F or 3F smokeless powder "containment" issues for those of us who don't have cans of 4F BP available at the local sidewalk sale. (everybody has real BP but will tell you all kinds of reasons you're not technically supposed to have it or legally buy it)

Also, why not volume in lieu of weight? A gram of 4F BP is a different size scoop than a gram smokeless powder I would imagine.

Once again, not trying to be a (_), just adding some thoughts while we're figuring how to or how not to stretch recovery harnesses.
 
I let this thread go because it got off on differneces of opinions about ejection charges. Now, can I ask again a different way? I have a 96 inch long rocket. It will use dual deploy. Using the "standard" formula of 4-5 times the length, I get a cord length of 40 feet. Some of you have said to use that length for both cords. What would you do?
 
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I let this thread go because it got off on differneces of opinions about ejection charges. Now, can I ask again a different way? I have a 96 inch long rocket. It will use dual deploy. Using the "standard" formula of 4-5 times the length, I get a cord length of 40 feet. Some of you have said to use that length for both cords. What would you do?

After a while you get diminishing returns on the length. I'd go closer to 4X the length. So 30' or so on each section should be fine.
 
What other kinds of failures are there?

Entanglements, not providing enough time for a main deploy, forgetting to attach a quicklink, parachute too small to do the job, bulkhead ripping out from a harness that's too long....

-Kevin
 
I flew my Big Nuke on April 30th in Amesbury, MA on a J350. I had 25 feet of tubular nylon on each end for recovery. I also had 3 grams of BP on each end. If you were to watch the video of the flight, it would be up for debate as to how the eyebolt would come slamming back to puncture a small repairable hole in the airframe. I am going with too much BP as it was observed that when the drouge charge went off, the 25 feet of tubular nylon was extended taut almost immediately. For its next flight which will be at NERRF, I am stepping back the charges to 2 grams as I have used that amount on 5.5" airframes before and everything separated fine. Back to the topic. I use longer lengths on my larger projects to keep everything as far apart as I can.

https://www.youtube.com/watch?v=4MLi28o6eiY
 
I let this thread go because it got off on differneces of opinions about ejection charges. Now, can I ask again a different way? I have a 96 inch long rocket. It will use dual deploy. Using the "standard" formula of 4-5 times the length, I get a cord length of 40 feet. Some of you have said to use that length for both cords. What would you do?

I would use 30 feet for the drogue and 30 feet for the main..
 
I let this thread go because it got off on differneces of opinions about ejection charges. Now, can I ask again a different way? I have a 96 inch long rocket. It will use dual deploy. Using the "standard" formula of 4-5 times the length, I get a cord length of 40 feet. Some of you have said to use that length for both cords. What would you do?

I have a rocket of amost exactly that size, 4" x 92". I use 18' of TN on the drogue side and 15' on the main side. It's had 11 flights and no issues with how anything deploys.
 
My Loc Graduator modified for dual deploy coming down the shock cords were 16ft drogue and main.Way too long and not needed,but it looked cool. :) The rocket is only 53 inches tall and should have had 22ft of shock cord by the math,but,that was what I felt comfortable using and it was a picture perfect drogue and main deploy with no damage to the rocket.

garduatorddrec.jpg
 
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May I suggest we all do a little reading up on Achimedes (pressure and area/volume) and Newton (bodies in motion, momentum and inertia). The mathematics is compex but the answers are definitive. Or we could all continue to speculate, speculate, speculate , conjecture , anecdote, speculate, conjecture, indirect inferrence, contrary anecdote and speculate.
By the way, this last statement is tounge in cheek humor.:D
 
Generally on 3" diameter rockets and larger, I will go 30' on the drogue cord, 15-20' on the main. The drogue cord is long because I want them as far apart as possible at apogee and on the way down. The main is slightly shorter because it's not as necessary to get them apart, but I want to give the chute plenty of space to get away from the tube.

This combined length is nice as well, because it almost always assures when I hang one in a tree, I have something to pull on...
 
I let this thread go because it got off on differneces of opinions about ejection charges. Now, can I ask again a different way? I have a 96 inch long rocket. It will use dual deploy. Using the "standard" formula of 4-5 times the length, I get a cord length of 40 feet. Some of you have said to use that length for both cords. What would you do?
OK, we'll start over. I would use 40-50 feet for the apogee event and about 10 feet for the main. First flight relatively low and drogueless to see free fall dynamics. Add a small drogue near the payload section if the fin can acts as a drogue or tries to streamline in on the first flight.

This is my Goblin on it's first flight showing that a drogue is needed.
[YOUTUBE]nXwzRN6Ebjk[/YOUTUBE]
 
This combined length is nice as well, because it almost always assures when I hang one in a tree, I have something to pull on...
Long cords have saved a few of mine too :wink: although I put most of the length on the apogee end :)
 
May I suggest we all do a little reading up on Achimedes (pressure and area/volume) and Newton (bodies in motion, momentum and inertia). The mathematics is compex but the answers are definitive. Or we could all continue to speculate, speculate, speculate , conjecture , anecdote, speculate, conjecture, indirect inferrence, contrary anecdote and speculate.
By the way, this last statement is tounge in cheek humor.:D

I could read up on Archimedes, but I don't see how it would prove useful. Buoyancy effects tend to be fairly small for rockets...

:p
 
With more cord comes a longer time it takes to fully deploy a chute.

On my 5" x 41lb rocket with a full L motor: I used 35-40 ft on the booster with 15-20 ft of 1" TN for the main. The main was set for 950 and 750 ft.

The main fully deployed just above the tree line, which is about 50-75ft above the ground.

With a rocket traveling 50-80 fps on the way down, you better increase you main altitude setting(s).
 
With more cord comes a longer time it takes to fully deploy a chute.

On my 5" x 41lb rocket with a full L motor: I used 35-40 ft on the booster with 15-20 ft of 1" TN for the main. The main was set for 950 and 750 ft.

The main fully deployed just above the tree line, which is about 50-75ft above the ground.

With a rocket traveling 50-80 fps on the way down, you better increase you main altitude setting(s).

Yep, even more important when using a deployment bag, which I do with all my big chutes in big rockets--but it adds more time to gracefully deploy. I usually set my main for 1200 and backup at 900.
 
I let this thread go because it got off on differneces of opinions about ejection charges. Now, can I ask again a different way? I have a 96 inch long rocket. It will use dual deploy. Using the "standard" formula of 4-5 times the length, I get a cord length of 40 feet. Some of you have said to use that length for both cords. What would you do?

The Bald Moms Rock extreme darkstar (sorta) was 94" tall, we used 40 feet in the main compartment, and 45 feet in the drogue.



Braden
 
That's excessive though. You really don't need that much. My L3 had less cord than that.
 
My L3 has 5 feet of 2 inch tubular nylon connected to 30 feet of 1 inch tubular nylon on both ends. I have my main set for 700 feet with a backup of 500 feet. I understand the concerns regarding time for chutes opening with longer cords. I have it so that the chute is the first to be ejected, if that makes sense. I have the chute sitting on top of all the shock cord.
 
I could read up on Archimedes, but I don't see how it would prove useful. Buoyancy effects tend to be fairly small for rockets...

:p

Quite correct, sir. Is it Avagadro who did work with gas pressures? Or maybe Boyle? I guess all the single malt whiskey I have run through the sytem has created a fragmented primary memory drive.:blush:
 
Our 12', 6" 53 pound USLI rocket this year had 16' at apogee and 12' on the main. My L3 rocket with a similar setup had 15' at apogee and 5' on the main. Both were free-bagged.
On the USLI rocket, on one of the flights, we flew 52' of cord on the main to put the rocket body on the ground if the parachute got hung up in a tree (it was windy, and the trees were kinda close). This resulted in shock loads that were ~4 times the normal at main deployment, as seen from the accelerometer data. This was the result of the rocket basically going into free-fall for about 1.5 seconds after the main was released, so it could get to the end of the cord before being stopped abruptly when it reached the bottom with the main open.

I've flown a 5.5" booster stage with the parachute attached directly to the eye bolt before too (no room for shock cord). This was with a deployment bag and an R12. Worked all 3 times it was flown, and there's nothing special about why it worked, it just isn't necessary to fly extra long shock cords.

I've seen more tangles from long stretches of cord getting hung up than anything but bad parachute packing. Take a look at how NASA recovers SRB's sometime. If they even use cord, it's minimal at best.
 
Ouch that's allot!

The longer cord will also increase chances of it tangling.
You are also adding unnecessary weight as well...
Mine will be using the stock TN from Wildman for the booster and a shorter 15ft that I made from the same kind of 9/16" TN for the payload section.

Your only key benefit will be: if it lands in a tree it might be a little closer to the ground..


JD

The Bald Moms Rock extreme darkstar (sorta) was 94" tall, we used 40 feet in the main compartment, and 45 feet in the drogue.



Braden
 
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Ouch that's allot!

The longer cord will also increase chances of it tangling.
You are also adding unnecessary weight as well...
My will be using the stock TN from Wildman for the booster and a shorter 15ft that I made from the same kind of 9/16" TN for the payload section.

Your only key benefit will be: if it lands in a tree it might be a little closer to the ground..


JD

As for tangling, it could potentially on the way down, but, the shock cord is folded in a "z" pattern and a loose rubberband is put around it, so it shouldn't get tangled on the ride up, or at deployment. We aren't too much for extreme altitude, so we aren't concerned as far as the weight goes.

Braden
 
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I have also done small bunches of loops for my shockcord as well. I am not using rubber bands though. I am using one wrap of masking tape.
 
ROCKET_SHOCKCORD.pngI guess it's a balance between sufficient length of shock cord to absorb the ejection charge without bouncing components together vs long chords which potentially result in breakage due to the extra velocity before hitting the end of the chord. To me the discussion is not about the total length of the chord but the separation you want to achieve between components. If we imagine the two stage process in slow motion then a common method for dual deploy at apogee is that the the payload nosecone combination ejected from the booster by the apogee charge and a small drogue pops out to slow down the booster, the nosecone/payload combination heads out and down in an arch until it swings below the booster on the shock cord. So the key at the booster stage is the separation between the tail of the booster and the top of the payload. I allow 9 feet for this on both my 10ft and 8ft rockets. Reducing the separation will increase the possibility of the booster hitting the payload and increasing it will increase velocity and risk of breakage. At the main altitude the nosecone is forced down away from the rest of the rocket pulling out the main chute. Typically the combination is drifting in the wind. The drag created by the opening chute slows the nosecone and the booster carries on to rotate around to below the the payload bay. Again you don't want the nosecone, payload and booster to hit each other so the key is the separation between the tip of the nosecone hanging from the main and the top of the booster which I have set at as 6-7 feet. If you draw a diagram of your rocket hanging under drogue and then under main including the length of nosecone, payload and booster you can work out the total length of your shock cord and then add the separation required to get the total length for each stage. My shock total cord length works out at about 5 times the total length of the rocket. To make it easier to calculate I have created a Google spreadsheet you can copy from here and plug in your own lengths. https://docs.google.com/spreadsheet/ccc?key=0AixSGBtMr5PfdGhlMTRtN1JhdUNubVA5UF9BcFcxSkE&usp=sharing
 
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Would you email me a copy of this spreadsheet? The online version does not allow data entry and I'm not seeing any formulas. Thank you.

The member you are addressing has made a total of three posts on this forum: two in 2011 and one in 2013.... I don't think they're going to be getting back to you--sorry.


Later!

--Coop
 
Good point, but I can always hope. In the meantime, I revised his online spreadsheet - embedded the graphic, converted meters to inches & feet, and made a few assumptions. If anyone would like to have a copy, or make a comment, you can find it here.
 
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